Clarisse Huguenard

Unified representation of MQMAS and STMAS NMR of half-integer quadrupolar nuclei

Abstract High resolution solid state NMR has been achieved for nuclei that exhibit quadrupolar interaction to the second order by MQMAS and more recently by STMAS. Both methods extend to several variants, 3QMAS, 5QMAS, … for MQMAS as well as ST 1 MAS, ST 2 MAS,… for STMAS. These experiments contain the same information content i.e. anisotropic second-order powder pattern and isotropic terms. A unified representation allows us to display the 2D spectra independently of the method used. Direct comparison of any such spectra is therefore straightforward and does not need any further renormalization.

3QMAS of three aluminum polycations: space group consistency between NMR and XRD

Abstract Three aluminum polycations, Al13 ϵ-Keggin Johansson-type, Al13 Mogel-type and Al30 δ-Taulelle-type have been measured by 27Al solid state NMR, using MAS and 3QMAS spectroscopies. The number of inequivalent aluminum obtained by these high resolution NMR experiments leads to a comparison with their respective XRD results obtained on single crystal. In all three cases, the inequivalent sites measured by NMR and deduced from the XRD measurement show apparent discrepancies. It is, therefore, shown that the choice of the space group has to be made in such a way that both diffraction and NMR measurement can be brought to a consistent description of the sample considered. In the case of Al30 δ-Taulelle, a new refinement of diffraction results has been performed changing the space group from C2/c to Cc leading to consistent results between XRD and NMR.

Molecular templating using titanium(IV)(oxo)alkoxides and titanium(IV)(oxo)aryloxides

Crystalline materials obtained by reacting titanium(IV) isopropoxide (1) with widely different ligands such as 1,3-propanediol, 2,2′-biphenol or triphenylacetic acid using acetone as a solvent have been characterised using single-crystal X-ray diffraction, 1H, 13C and 17O NMR spectroscopies and theoretical modelling. In the case of the previously described [Ti3(μ3-O)(OPri)4(μ-OPri)3{Me2C(O)CHC(O)CH2C(O)Me2}] complex (2), the importance of the simultaneous use of HSQC, HBMC and ROESY techniques for a full spatial attribution of all resonances was demonstrated. The optimised molecular structure of (2) and its associated dynamics have also been used to demonstrate that more shielded 1H and 13C resonances and shorter relaxation times were expected for more rigid bridging OR groups relative to terminal ones. A templating effect linked to the steric decompression that may occur when the rather constrained molecular structure of [Ti3(μ3-O)(μ3-OPri)(μ-OPri)3(OPri)6] (3) meets acetone molecules in their enol form, (3) + 3 CH2C(OH)CH3 → (2) + 3 HOPri, is proposed, explaining why exactly three acetone molecules should undergo aldol condensation and why new complexes such as [Ti3(μ3-O)(μ3-OH)(OPri)6(μ-OPri)3] (4) or [Ti3(μ3-O)(μ-OOCPh3)2(μ-OPri)3(OPri)5] (5) may be isolated and characterised.

A remarkable solvent effect on the nuclearity of neutral titanium(IV)-based helicate assemblies.

The spontaneous self-assembly of a neutral circular trinuclear Ti(IV) -based helicate is described through the reaction of titanium(IV) isopropoxide with a rationally designed tetraphenolic ligand. The trimeric ring helicate was obtained after diffusion of n-pentane into a solution with dichloromethane. The circular helicate has been characterized by using single-crystal X-ray diffraction study, (13) C CP-MAS NMR and (1) H NMR DOSY solution spectroscopic, and positive electrospray ionization mass-spectrometric analysis. These analytical data were compared with those obtained from a previously reported double-stranded helicate that crystallizes in toluene. The trimeric ring was unstable in a pure solution with dichloromethane and transformed into the double-stranded helicate. Thermodynamic analysis by means of the PACHA software revealed that formation of the double-stranded helicates was characterized by ΔH(toluene)=-30 kJ mol(-1) and ΔS(toluene)=+357 J K(-1)  mol(-1) , whereas these values were ΔH(CH2 Cl2 )=-75 kJ mol(-1) and ΔS(CH2 Cl2 )=-37 J K(-1)  mol(-1) for the ring helicate. The transformation of the ring helicate into the double-stranded helicate was a strongly endothermic process characterized by ΔH(CH2 Cl2 )=+127 kJ mol(-1) and ΔH(n-pentane)=+644 kJ mol(-1) associated with a large positive entropy change ΔS=+1115 J K(-1) ⋅mol(-1) . Consequently, the instability of the ring helicate in pure dichloromethane was attributed to the rather high dielectric constant and dipole moment of dichloromethane relative to n-pentane. Suggestions for increasing the stability of the ring helicate are given.

Spontaneous symmetry breaking during self-assembly of a double stranded biphenolate-based Ti(IV)-helicate.

A binuclear Ti(IV)-based helicate synthesized from a symmetric tetrahydroxyheptaphenylene strand was self-assembled in solution and shown to undergo a spontaneous head-to-tail differentiation according to single-crystal X-ray diffraction.

Fluorine-19 NMR from retrosynthesis to NMR crystallography

Abstract Recently, the need for more accurate structure description that will allow to analyze possible retrosynthesis of microporous compounds has led us to use and develop new structural strategies to access structure determination by solid state NMR techniques. At first, an example, AlPO 4 -CJ2, between crystallogenesis and crystal structure is explored. AlPO 4 -CJ2 has been studied by hydrothermal in-situ NMR. A soluble species exists in supersaturated liquid as well as the product of redissolution of AlPO 4 -CJ2. This species, the prenucleation building units (PNBU) is different from structural buildings units (SBU) the sub-ensembles of the crystal. How this PNBU gets into the structure and becomes therefore a structural building unit (SBU) is our main question. To answer it, an HETCOR experiment between fluorine and aluminum has been performed. This experiment shows that when the PNBU enters the solid, it forms at the same time than growing the solid, a fluorinated bridge. On ULM-18 after structure elucidation positioning hydrogen, a dynamic charge compensation mechanism is proposed for the solid formation. On TMPGaPO, a set of 2D experiments using radio frequency driven dipolar recoupling (RFDR) on 19 F , Double Quanta on 31 P and HETCOR between 19 F and 31 P allowed analyzing the topology of the network.